A number of polyurethane foams have been produced from aqueous polyurethane latices or dispersions. For example, U.S. Pat. No. 3,461,103 and U.S. Pat. No. 3,479,310, along with Great Britian Pat. No. 1,076,688, disclose churning emulsifier-free ionic polyurethane dispersions into foamed plastics. U.S. Pat. No. 3,989,869 to Neumaier et al. discloses frothing an emulsifier-free nonionic aqueous polyurethane dispersion and casting and drying the froth into a foam. The Neumaier et al. method employs polyurethane dispersions which have particles less than 1 micron in diameter, solids contents of more than 45 percent by weight and dispersion viscosities in the 200 to 1200 centipoise range, measured prior to the addition of about 0.1 to about 10 percent by weight of a thickening agent. Neumaier et al. do not disclose the viscosity of the dispersion after the addition of a thickener, but explain that the method is an improvement over their own prior method in which ionomeric polyurethane dispersions were foamed without thickener and with the addition of foaming agents, stabilizers and crosslinking compounds.
The prior method cited in Neumaier et al. is the subject matter of U.S. Pat. No. 4,029,534 to Bocks et al., which teaches the preparation of foams from certain polyurethane dispersions which have specific macroscopic properties. More particularly, the dispersions taught as suitable for use by Bocks et al. (1) must have a solids content between 45 and 62 percent by weight; (2) must have a viscosity between 200 to 1200 centipoise; (3) must have particles of a diameter between 0.05 and 1.0 micron, preferably between 0.07 and 0.30 microns; (4) must have a solubilizing ionic group present in an amount between 2 and 40 milliequivalents per 100 grams of polyurethane solids; and (5) must yield a homogeneous foil (cast without foaming) having a tensile strength of at least 40 kg/cm.sup.2.
Unfortunately, the formulations and processes described in the prior art cannot always yield the necessary foam product and, more importantly, require high energy input during manufacture in order to process and shape the high viscosity starting materials. A need remains, therefore, for a flexible polyurethane foam sheet which has superior characteristics and which is further capable of manufacture from low viscosity admixtures containing dispersed polyurethane particles.